The invention relates to a device for reducing the vibrations of a structure along a vibration propagation line, in particular in a helicopter, as well as to a method for reducing the vibrations of a structure.
Cellular structures in helicopters are subject to considerable vibrations. These are caused for example by rotation of the main rotor and extend, inter alia, along an axis of vibration propagation inside the cabin. They are responsible, in particular, for cabin vibrations arising at the pilot""s seat, which are distracting and uncomfortable for the pilot.
Vibration absorbers are conventionally provided at the main rotor to reduce the vibrations, these being capable of transmitting static forces but filtering out vibrations. A vibration absorber of this type is known from EP 0519786 B1. This consists substantially of two coaxially arranged housings, between which resilient restoring means is arranged, which connects the two housings together.
The known vibration absorber has the problem of markedly increasing the total weight of the helicopter. In addition, vibration absorbers of this type are unable to operate optimally owing to the constantly changing rotational speed of the main rotor, since the known vibration absorbers are unable to adapt their vibration-absorbing abilities quickly enough to the varying vibration times. In addition, the structure is very complex.
It is therefore an object of the present invention to provide a device for reducing vibration in a structure, which device is of simple construction and with which it is possible effectively to reduce vibrations having constantly varying vibration times. In addition, the device is designed to be easily attachable at the location to be kept free of vibrations, e. g. the pilot""s seat in the cabin.
To achieve this object, the invention provides a device for reducing the vibrations of a structure along a vibration propagation line, in particular in a helicopter, in which two vibrating members of masses m1 and m2 are located at or near the ends of a spring, wherein the spring and/or the vibrating members are so constructed that the resonance characteristics may be variably adjusted within a range decisive for the reduction of vibrations.
Devices according to the invention can reduce the vibrations arising within a broad frequency spectrum and can be constructed to adapt automatically and rapidly to constantly changing frequencies. The device can be made compact and thus suitable for confined spaces, and may in particular be used in a helicopter, where it may easily be mounted within the cabin, e. g. in the vicinity of the pilot""s seat, in order to keep this area free of vibrations. The structure in question can then be constituted by the floor in the vicinity of the seat, or even the seat or seat mounting itself. The spring can be mounted on the structure either directly or indirectly, by fixing it in a housing which in turn is fixed to the structure. The fixing can be by way of an interference fit, for example.
The assembly preferably includes means provided on the vibrating members and in the center, i.e. at the base point, of the spring, for adjusting the resonance characteristics of the device, said means being capable of sensing the movement of these components relative to the surrounding environment. Depending on the phase difference between the vibration of the vibrating member of mass m1 and the vibration of the base point on the one hand and the phase difference between the vibration of the vibrating member of mass m2 and the base point on the other, correcting variables derived therefrom modify the resonance characteristics of the spring/mass system until the absorption of the vibrations by the device, i.e. the reduction of the vibrations of the structure, reaches its maximum.
To determine the phase difference between the movements of the vibrating members and the movement of the base point, the device preferably comprises phase detectors having a control output, which, with the correcting variables output at that point, varies the resonance characteristics as a function of the phase differences detected by the detectors. Thus, a control circuit is formed by means of the phase detectors which senses the vibrations of the spring/mass system at the various positions using the means for measuring the movement, compares them in the phase detectors and modifies the resonance characteristics by means of the control outputs until the reduction of the vibrations of the structure reaches an optimum level. The phase detectors are advantageously multiplexers with control outputs.
It is also preferable for a comparison between the amplitude of the movements of the vibrating members to be carried out, in order to modify the resonance characteristics of the spring/mass system as a function of the amplitude difference between the movements. The amplitude differences between the two vibrating members are reduced to a specified range.
The movement-measuring means preferably take the form of acceleration sensors. However, it is perfectly possible for other detectors to be used, with which it is possible to sense the accelerated movements of the vibrating members, i.e. the vibrations. In the event of simultaneous measurement of the movements of the vibrating members for determining the phase difference and measurement of the movement of the vibrating members for determining the amplitude difference, only one means is attached to each vibrating member for movement measurement, meaning that only one acceleration sensor is provided for both measurements and the signals of the acceleration sensors are used both for phase comparison and for amplitude comparison.
Amplitude controllers having a control output are preferably used in the device to determine the amplitude difference between the movements of the two vibrating members of the masses m1 and m2. The correcting variables emitted via the output as a function of the amplitude difference detected by the amplitude controller then modify the resonance characteristics of the spring/mass system. The control circuit formed here by means of the amplitude controller senses the movements of the vibrating members and compares the amplitudes of the vibrations with a set value or set value interval, whereupon the control output of the amplitude controller modifies the resonance characteristics until the measured amplitude difference reaches the specified value or lies within the specified interval. The amplitude controller is advantageously a comparator with control output.
For simultaneous adjustment of the resonance characteristics by means of the phase difference between the movements of the vibrating members and the base point of the spring and adjustment via the amplitude difference between the movements of the vibrating members, the device preferably includes a logic component which receives the correcting variables from the two control circuits and combines them to form overall correcting variables for modifying the resonance characteristics.
Modification of the resonance characteristics of the spring/mass system may be effected by modifications to the spring or the spring material, or the clamping of the spring, in order to have a direct effect on or to change the spring constant and thus the resonance characteristics of the spring. However, it is advantageous for at least mass components xcex94m1 and xcex94m2 of the vibrating members of the masses m1 and m2 to be mounted so as to be displaceable along the axis of vibration propagation. This displacement of the mass centers of the vibrating members has the effect of modifying the vibration characteristics of the spring/mass system of the device, which results in a change to the resonance characteristics. It is of course also possible to displace the entire vibrating members of the masses m1 and m2 along the axis of vibration propagation.
Any desired means may contribute to the displacement of the mass components xcex94m1 and xcex94m2, but it is preferred that adjusting devices move the mass components along the axis of vibration propagation in such a way that the centers of gravity of the vibrating members are displaced outwards or inwards in accordance with the position of the mass components xcex94m1 and xcex94m2 predetermined by the adjusting device. The same is of course true for displacement of the entire vibrating members of the masses m1 and m2.
The adjusting devices are advantageously stepper motors, which effect displacement of the mass components xcex94m1 and xcex94m2 or the entire vibrating members of the masses m1 and m2 by means of spindle drives. By using stepper motors, the respective position of the motor is known and may be specifically changed. The spindle drives convert the rotational movement of the stepper motors into linear displacement of the mass components or the entire vibrating members along the axis of vibration propagation. Specific alteration of the position of the motors thus displaces the mass components by a predetermined distance in the direct direction.
The invention also relates to a method for reducing vibrations along an axis of vibration propagation, particularly in a helicopter.
A further object of the present invention is to provide a method for reducing vibrations along an axis of vibration propagation, with which, in particular in a helicopter, vibrations having varying vibration times are reduced effectively and simply.
According to the method of the invention the movements of two vibrating members of the masses m1 and m2 attached at the ends of a spring and the movement of the central area of the spring, i.e. the base point, are sensed and the resonance characteristics of the spring and vibrating member system are modified by means of correcting variables as a function of the phase difference between the movements on the mass side and the movement on the base point side until a target phase difference value is reached.
It is advantageous for the target value of the phase difference to be set at 90xc2x0, since, under normal test conditions, maximum vibration reduction is achieved when this is the case. However, circumstances may also arise which make it necessary for the target value of the phase difference to be set at a value other than 90xc2x0.
The movements of the vibrating members are preferably also sensed and the resonance characteristics modified by means of correcting variables as a function of the amplitude difference between the movements of the vibrating members, until the measured amplitude difference reaches a set value or lies within a set value interval.
Although it is possible, in the event of simultaneous adjustment of the resonance characteristics by means of measurement of the phase difference between the movements of the vibrating members and the movement of the base point of the spring and by means of measurement of the amplitude difference between the movements of the vibrating members, for the adjustments to be carried out in succession, it is advantageous, however, for the correcting variables resulting from the various measurements to be combined into overall correcting variables, whereby the two adjustments are prevented from influencing each other, which may happen when different modifications of the resonance characteristics are run in succession.
If the various correcting variables are combined to provide an overall correcting variable, adjustment of the resonance characteristics using measurement of the phase difference is advantageously interrupted for the entire period of adjustment using measurement of the amplitude difference. The pending adjustment process using the measured phase comparison is interrupted and not run jointly therewith, since adjustment using the amplitude comparison may influence phase comparison adjustment; the latter adjustment is thus resumed when amplitude compensation is complete.